Typically, when exploration or servicing must occur in deep water, ROVs are used for access to a site and for completion of a variety of different tasks. ROVs are generally self-propelled for proper positioning, and include manipulation linkages to allow such activities as turning valves to be accomplished by the ROV. The ROV typically also has video equipment and lighting to allow personnel at the surface to better direct its movements for proper positioning to accomplish a specific job. ROVs are frequently deployed using structures known as tether management systems (TMS). A deployed tether management system can be either a cage type with the ROV stored inside it or a top hat type with the ROV stored below it. The TMS with ROV is lowered from a vessel at the surface with a winch system. When the TMS reaches close to the seabed, the ROV is actuated to disengage from the TMS, and is thereafter directed to the work site location. The ROV is tethered to the TMS to facilitate its operation by the transmission of power and signals to the ROV from the surface through the TMS.
One of the problems in deploying ROVs is the space required on the surface vessel to house the TMS and ROV. Typically, a surface vessel will include a single ROV with a TMS to accomplish a particular task. If mechanical or other difficulties ensue with regard to the ROV, there can be significant delays before a replacement unit can be brought to the surface vessel. A replacement unit would also require additional deck space.
As previously stated, ROVs accomplish a variety of different functions. In many applications, the purpose of the ROV is really to illuminate and transmit video to the surface for monitoring of particular subsea equipment or condition. It is therefore one of the objects of the present invention to allow the ability to perform certain tasks which require an ROV, even if the main ROV on the surface vessel experiences operational difficulties. This objective of the present invention is resolved by configuring a TMS to not only accept a main ROV, but also a smaller mini ROV, preferably housed directly below the cage type TMS or housed within the top hat type TMS. Accordingly, if problems ensue with the main ROV, certain functions can continue to be accomplished with the mini ROV until a replacement ROV is delivered to the surface vessel. The configuration into a compact package is another objective of the present invention. Depending on the space availability of the particular application, the mini ROV can have some or most, if not all, the capabilities of the original ROV. Practically, in most applications, the mini ROV will have substantially fewer capabilities than the principal or main ROV.
Illustrative of use of ROVs in the prior art are U.S. Pat. Nos. 4,010,619, 4,686,927, 4,721,055 and 5,069,580. These and other benefits of the present invention will be readily apparent to those skilled in the art from a review of the description of the preferred embodiment below.